1. Field of the Invention
The present invention relates to a magnetic toner for use in image forming methods such as an electrophotographic method, an electrostatic recording method, a magnetic recording method and a toner jet system recording method.
2. Description of the Related Art
As an electrophotographic method, a large number of techniques have been known. Generally, a photoconductive material is used, and an electrostatic latent image is formed on a photosensitive member by any of various means. Then, the electrostatic latent image is developed with a toner, and the resultant toner image is subsequently transferred to a transfer material such as a paper, if necessary. Thereafter, the transferred toner image is fixed by heating, pressurizing, heating and pressurizing, a solvent vapor or the like to obtain a subject image. The developer which has not been transferred and thus left on the photosensitive member is removed by any of various methods to clean the photosensitive member. The above-described steps are then repeated.
In this technique, as the developing method, a one-component developing process is preferably used, because in one-component developing process, a developing unit having a simple structure is used with less troubles, and it has a long life and its maintenance is easy.
In such a one-component developing process, the quality of a formed image depends largely on the performance of a magnetic toner to be used. In the magnetic toner, a considerable amount of magnetic particles in the state of fine powder is mixed and dispersed, and part of the magnetic particles are exposed on the surface of the magnetic toner. Therefore, the kind of magnetic particles has an influence on fluidity and frictional electrification of the magnetic toner, and consequently, on various properties such as developing properties and durability required in the magnetic toner. Accordingly, with regard to the magnetic particles contained in the magnetic toner, various proposals have heretofore been made.
Furthermore, in order to improve the properties of the magnetic particles, preparation methods have heretofore been investigated in which, during a reaction for producing the magnetic particles, silicon is added to a reaction system. For example, there has been a method which comprises the successive steps of adding a silicon component to a ferrous salt solution, mixing the solution with 1.0 to 1.1 equivalents of an alkali to iron, carrying out an oxidative reaction while a pH is maintained at 7 to 10, adding 0.9 to 1.2 equivalents of iron to the original alkali during the reaction, and carrying out the oxidative reaction while the pH is maintained at 6 to 10, to thereby obtain the magnetic particles (e.g., see Japanese Patent Publication No. 8-25747, and Japanese Patent Application Laid-Open No. 5-213620). Furthermore, there has been another method in which when an oxygen-containing gas is passed through an aqueous ferrous salt solution containing ferrous hydroxide colloid obtained by allowing Fe2+ to react with 0.80 to 0.99 equivalent of an alkali hydroxide to produce the magnetic particles, 0.1 to 5.0 atom % of a water-soluble silicate in terms of Si to iron is added to the aqueous solution, and then, a two-stage reaction is performed to obtain the spherical magnetic particles (e.g., see Japanese Patent Publication No. 3-9045).
Furthermore, as to a magnetic iron oxide for use in the magnetic toner, there are known magnetic particles comprising 1.7 to 4.5 atom % of silicon in terms of Si to iron and a magnetic iron oxide containing, as a metal element other than iron, 0 to 10 atom % of one or more metal elements selected from the group consisting of Mn, Zn, Ni, Cu, Al and Ti to iron (e.g., see Japanese Patent Application Laid-Open Nos. 9-59024 and 9-59025). When such magnetic particles are used, magnetic properties and charging properties of the magnetic toner are improved. However, when the above-described metals are simply added, both the developing properties and the image quality in a high-speed developing system are not simultaneously sufficiently satisfied, and room for improvement is left.
Furthermore, there are known magnetic particles which contain a silicon component continuously from centers to surfaces of the magnetic particles and in which the silicon components are exposed on the surfaces of the particles and the particles are coated with a metal compound having at least one metal element selected from the group consisting of Zn, Mn, Cu, Ni, Co, Cr, Cd, Al, Sn, Mg and Ti bonded to the silicon component (e.g., see Japanese Patent Application Laid-Open No. 11-157843). However, when such magnetic particles are used for a long term especially in the high-speed developing system, the deterioration of the image quality and the developing properties cannot be restrained, and therefore the above technique has respects, which should be further improved.
There are known magnetic particles which regulate a content of one or more metal elements selected from the group consisting of Mn, Zn, Ni, Cu, Co, Cr, Cd, Al, Sn and Mg on the basis of iron element, a content of silicon element, a content ratio of silicon element, existing up to an iron element solubility of 20 mass %, and a content ratio of the silicon element, existing up to an iron element solubility of 10% (e.g., see Japanese Patent Application Laid-Open Nos. 11-316474, 11-249335, and 11-282201). According to such magnetic particles, various metals are contained in a magnetic iron oxide, and distribution of silicon in the magnetic particles is regulated, whereby an improved environmentally stabilized effect is obtained. With regard to durability in the high-speed developing system, however, further improvement is required.
Moreover, there are known magnetic particles in which a silicon component is continuously exposed from centers to surfaces of the magnetic particles and whose shells are coated with a metal compound having at least one metal atom selected from the group consisting of Zn, Mn, Cu, Ni, Co, Cr, Cd, Al, Sn, Mg and Ti bonded to the silicon component and an aluminum component (e.g., see Japanese Patent Application Laid-Open No. 11-189420). In a magnetic toner having the magnetic particles, electric resistance and aggregation are satisfactory, however, saturated susceptibility remarkably deteriorates due to the increase of the silicon and aluminum components exposed on the surfaces of the magnetic particles, and a sufficient charging stability is not provided for the magnetic toner yet. Thus, a problem to be solved resides in a change of magnetic properties which depend on the thickness of a coating film of the magnetic particles.
Furthermore, there are proposed magnetic particles containing magnesium (e.g., see Japanese Patent Application Laid-Open No. 6-144840); magnetic particles containing aluminum and silicon (see Japanese Patent Application Laid-Open Nos. 5-72801, 5-213620, 7-175262, 7-239571, 7-110598, and 11-153882); magnetic particles containing zinc (e.g., see Japanese Patent Application Laid-Open Nos. 8-50369, 8-101529, 7-175262, 8-48524, 8-208236, and 8-208237); and magnetic particles containing silicon and other elements (e.g., see Japanese Patent Application Laid-Open Nos. 11-157843, 11-189420, and 11-316474).
When the magnetic toners containing the above-described elements are used, satisfactory developing properties are obtained. However, there are demands for further improvements in the developing properties and the durability in a case where the toner is applied to a developing system at a high speed or a developing system having a simple constitution. Specifically, there is further room for improvement concerning various problems caused by fluidity or charging properties of the magnetic toner, such as sleeve blotch, image scattering, and ghost, or the drop of the developing properties by a temperature rise in a machine.
Moreover, there are known magnetic particles characterized by a hexahedron or octahedron shape, in which a layer containing many silicon elements is formed on a nucleus surface formed by an iron atom uniformly mixed with a bivalent metal atom selected from the group consisting of Zn, Mg, and Mn (e.g., is see Japanese Patent Application Laid-open No. 8-50369). As to a magnetic toner using the magnetic particles, there is not any fog from a low-speed copying machine to a high-speed copying machine, and a high-density copied image is obtained. The toner is not influenced by any environmental fluctuation, and is superior in durability. However, there has not been any improvement from a viewpoint of retention of blackness or reduction of consumption.
As described above, the magnetic particles for use in the magnetic toner have not been improved in order to obtain a superior charged amount, retain the environmental stability, and suppress image defects such as blackness and ghost. In present situation, there is also left room for study concerning the reduction of the consumption of the toner formed from the magnetic particles.